Raymond Y. Huang | Searcharxiv

arXiv2026

Unveiling neuronal microstructure in the human brain in vivo with time-dependent radial diffusivity in MRI

Diffusion time-dependence, defined as variations in diffusivity and/or diffusional kurtosis with diffusion time, has emerged as a valuable non-invasive imaging marker for characterizing tissue microstructural features, such as cell size, density, packing disorder, and membrane permeability. In white matter, diffusion time-dependent changes between the short diffusion time and long diffusion time in radial diffusivity (RD), defined as the diffusivity perpendicular to fiber tracts, were demonstrated to correlate strongly with mean axon diameter in ex vivo spinal cord tissues, and to reveal demyelination in mouse corpus callosum. Despite their potential to non-invasively unveil neuronal microstructures to improve the assessment and targeted therapy of neurological diseases, these novel image contrasts obtained at short diffusion times using oscillating gradient spin echo (OGSE) have only recently become feasible for human in vivo studies with high-performance gradient MRI systems. In this preliminary study, we characterized time-dependent RD with OGSE encoding in the human brain in vivo. The change in radial diffusivity between short diffusion time and long diffusion time (delta_RD) consistently exhibited high values in the corticospinal tract, indicating high sensitivity of delta_RD to large axon diameter in human brains. Imaging at a high OGSE frequency of 100 Hz and a moderate b-value of 800 s/mm2 produced the highest delta_RD in the corticospinal tract. This study established a baseline for future investigations of neuronal microstructural alterations in neurological disorders and diseases.

physics.med-ph↗

arXiv2025

Human brain high-resolution diffusion MRI with optimized slice-by-slice B0 field shimming in head-only high-performance gradient MRI systems

The purpose of this study is to propose a brain tissue-selective, optimized slice-by-slice B0 field shimming for high-resolution brain diffusion MRI. We incorporated actual gradient fields of X, Y, and Z gradient coils in the calculation of the shimming coefficients in dynamic slice-by-slice B0 field shimming to minimize B0 field inhomogeneity (i.e., Delta B0) in deep-learning segmented brain tissues. Diffusion MRI with oscillating gradient spin echo (OGSE) at 55 Hz and pulsed gradient spin echo (PGSE) (approximated at 0 Hz) were obtained in phantoms and healthy volunteers using a head-only high-performance gradient 3T MRI system. In each diffusion MRI acquisition, standard static volumetric shimming and the proposed shimming method were applied separately, and mean/axial/radial diffusivities (MD/AD/RD) and fractional anisotropy (FA) were estimated. In phantom, the root-mean-square of Delta B0 in areas with high gradient nonlinearity was reduced by 7 Hz when incorporating actual gradient field in dynamic shimming. Compared to static shimming, dynamic shimming reduced root-mean-square of voxel displacement of each slice by a maximum of 5-10 voxels in single-shot EPI acquisition at 1-2 mm in-plane resolution in phantom, and a maximum of 3 voxels in human brains. Improved accuracy of MD/AD/RD/FA in the superior region of the brain, brainstem, and cerebellum were observed by applying dynamic shimming and/or two-shot EPI acquisition. MD(55 Hz)-MD(0 Hz) showed higher values in T2 FSE hypo-intensity region by applying dynamic shimming. We concluded that diffusion MRI with brain tissue-selective, dynamic slice-by-slice B0 effectively improves the accuracy of diffusivity characterization in high-resolution images.

physics.med-ph↗